The use of fluorescence indices for sensing the impact of abiotic and biotic stresses in agricultural crops is well documented in the literature. Pigment fluorescence gives a precise picture about the plant physiology and its changes following the occurrence of stresses. In general, alterations in such optical signals is caused either by the stress-induced accumulation of one or more fluorophores, or the degradation of specific molecules like chlorophyll. Unfortunately, many stresses might influence the fluorescence signature of leaves and plants in a similar way. Thus, one of the biggest challenges aiming the practical use of optical sensors is not only to detect the occurrence of stresses, but also to differentiate between different stress types. With this background we conducted our experiments using potted plants allocated inside a polytunnel to evaluate the potential of several fluorescence indices for the detection and differentiation of abiotic and biotic stresses at leaf level.

 

Two commercial sugar beet (Beta vulgaris L.) cultivars (Pauletta and Cesira) differing in their susceptibility to powdery mildew, were fertilized with two nitrogen levels, exposed to water deficit and/or inoculated with powdery mildew. Abiotic and biotic stresses were applied individually or in combination while control plants did not suffer from any stress. Changes in plant physiology were recorded with a multiparametric handheld fluorescence sensor (Multiplex, Force-A, France) and a fluorescence imaging system (Nuance CRI, Perkin-Elmer, USA) at two developmental stages. Moreover, morpho-physiological as well as analytical standard analysis such as osmotic potential and chlorophyll content served as reference.

 

In general, both cultivars showed a similar response pattern to the applied treatments, although the intensity of stress-induced modifications was genotype-specific. While significant changes in the osmotic potential were caused only by water deficit, the decrease in the chlorophyll concentration was caused by water deficit and powdery mildew as single or combined factors. Using the handheld fluorescence sensor, our results also indicate that the ‘Nitrogen Balance Index’ (NBI) and the ‘Simple Fluorescence Ratio’ (SFR) were sensitive parameters to indicate the impact of the stresses. Similarly, the ‘blue-to-far red fluorescence ratio’ (BFRR_UV) revealed significant alterations in the physiology of the sugar beet leaves. These results also confirm our results from a field experiment with focus on the nitrogen supply. At all, our work demonstrates that fluorescence indices might be used as single or combined indices for successful sensing of stresses. However, an overall and robust differentiation of the stress factors by using only one fluorescence ratio could not be accomplished. In the sum, our results clearly demonstrate the potential, but also some limitations, for using specific fluorescence indices as diagnostic tool in precision farming.